This invention deals with the subject of composite articles, including composite single crystals, and methods for producing the same. A brief description of grain boundaries will be helpful in appreciating the benefits obtained by the use of single crystals. A grain boundary is a region separating two grains of different crystallographic orientation. Differences of opinion exist in the metallurgical field regarding models for grain boundary structure and methods for interpreting grain boundary effects. One way of classifying grain boundaries is in terms of the difference in orientation between the grains. Low angle boundaries, those which separate grains of only slightly different orientation, may be analyzed as arrays of dislocations. Equations and models developed using this assumption have given results which agree well with experimental evidence. The amount of misorientation necessary for a boundary to be classified as high angle is not well established but is usually considered to be greater than about 5.degree.. Wholly satisfactory models have not been developed for high angle grain boundaries. On the basis of experimental evidence it is known that high angle boundaries have much greater mobility than low angle boundaries, and that in general, the mobility of grain boundaries increases as the angle separating the adjacent grains increases. Low angle boundaries are commonly referred to as sub-boundaries and in the remainder of this application the term grain boundary will mean high angle grain boundary unless otherwise indicated. As the angle between the adjacent grains increases the boundary region becomes increasingly disordered. Impurity atoms within the material will in general have different size and electronic properties than the base material. These impurity atoms will be attracted to and will fit better in the disordered region found in high angle grain boundaries. The concentration of impurity atoms at the grain boundary can be several orders of magnitude greater than the concentration of the same impurity atoms within the crystal itself. When such segregation occurs the properties of the grain boundary begin to assume the properties of the impurity atoms. For example, in nickel base superalloys sulfur is a deleterious impurity which segregates to the grain boundaries. At operating temperatures the sulfur in the grain boundaries degrades their mechanical properties and failure in sulfur bearing nickel base superalloys will generally occur by grain boundary failure. Impurity segregation is not a particular problem with low angle grain boundaries. Accordingly, low angle boundaries do not present the same problems in mechanical behavior as do high angle grain boundaries. Grain boundaries are also observed to have deleterious effects during high temperature oxidation and corrosion.
Attempts have been made to reduce these problems through the use of such techniques as directional solidification, described in U.S. Pat. No. 3,260,505, assigned to the assignee of the present application, which minimizes transverse grain boundaries, and through the use of single crystals, as described in U.S. Pat. No. 3,494,709, assigned to the assignee of the present application, to form complete small turbine parts such as blades and vanes. The results obtained from the directional solidification progress have been highly satisfactory but to date the incremental improvements obtained through the use of single crystal techniques have not been attractive in view of the added costs.
Another technique which has aroused interest in the field of high temperature alloys is the technique of growing crystals having an oriented microstructure or second phase. See for example, U.S. Pat. Nos. 3,793,010 and 3,528,808, both of which are assigned to the assignee of the present application. The conditions required to grow an oriented second phase structure include rigid controls on composition and it has been found that most alloys having the correct composition, for optimum mechanical properties, do not in general have adequate high temperature oxidation and corrosion resistance for demanding applications such as gas turbine engines.